1. Field of the Invention
The present invention relates to a plastic molding for use in, for example, an optical apparatus, such as a copier, a digital copier using a laser system, a laser printer, a facsimile machine, an optical apparatus for video camera, or an optical disk system, and a method and a mold assembly for producing the same. More particularly, the present invention relates to a plastic molding such as a plastic mirror or a plastic lens having a large and uneven thickness, and a method and a mold assembly for producing the same with high accuracy and low-cost.
2. Description of Related Art
An optical element such as a lens or a mirror is used in an optical unit of an optical apparatus, such as a copier, a printer, a facsimile machine, and so forth. In recent years, with the increase of demand for reducing costs of products, the base material of such an optical element has generally shifted from glass to plastics. Further, an optical element having a complicated aspheric surface is frequently formed for achieving multi-functions with a reduced number of optical elements. Such an optical element is generally designed to have a thick and uneven thickness distribution.
Such an optical element can be produced with low-cost and high mass-productivity using a method by which molten resin is injected into a cavity of a mold assembly or another method by which base-resin is inserted into a cavity of a mold assembly, even when producing a peculiar configuration.
For example, an injection molding method and a compression injection molding method are known. According to the known injection molding method, molten resin is injected into a cavity having a prescribed volume in a mold assembly that is controlled to have a temperature lower than a softening point of the resin. Then, the molten resin is cooled to solidify under a controlled pressure thereof. The solidified resin, or the resulting molding, is taken out by opening the mold assembly.
Generally, in the solidification process by cooling the molten resin in the cavity, a temperature or a pressure of the resin in the cavity is preferably intended to be uniform, in order to produce a plastic molding having a desired shape with high accuracy.
However, in producing a plastic molding such as a lens, the plastic molding is deteriorated due to differences of the amount of shrinkage within the volume being molded, which is caused by differences of cooling rates of the resin due to the uneven thickness. For example, as illustrated in FIG. 12, in producing a plastic lens 3 having a thick and uneven thickness, the plastic lens 3 actually formed may be caused to sink in a sinking portion. An exemplary view of such a sinking portion is illustrated in FIG. 13. Namely, in FIG. 13, a sinking portion 151 extends widely and reaches not only a side surface (non-transfer surface) 121 but also an optical surface (transfer surface) 111.
In order to overcome such problems related to sinking, a pressure of injected molten resin may be increased. In this case, the amount of the molten resin is thereby increased. However, the inward deformation of the plastic molding is also caused to increase. In particular, when the plastic molding is thick, optical properties thereof are deteriorated due to the large inward deformation of the thin portion.
According to the compression injection molding method, a transfer piece, which provides a portion of a cavity wall, is configured so as to be capable of moving inside a mold assembly. The transfer piece is advanced so as to follow the shrinkage of resin filled up in the cavity during a cooling process of the resin. For example, in producing a lens having an uneven thickness, each portion has a difference in amount of the shrinkage due to the uneven thickness in the longitudinal direction thereof. The transfer piece is advances in compliance with the shrinkage so as to apply a constant pressure to the resin. Therefore, the reduction of pressure during the cooling process is compensated according to the compression injection molding method.
Accordingly, the molten resin can be injected under a lower injection pressure than that of the above-mentioned injection molding method. However, the transfer piece is not always able to follow the shrinkage of resin due to uneven shrinkage that depends on thickness distribution of the molding. Therefore, portions of some moldings are still caused to sink.
From this point of view, other studies have been performed to overcome shortcomings of the injection molding method and the compression injection molding method. For example, according to a method disclosed in the Japanese Laid-Open Patent Publication No. 2-175115 or No. 6-304973, a molding is configured so as to sink in a surface other than a transfer surface of a mold assembly. The mold assembly includes a vent hole provided in a surface other than the transfer surface (mirror surface) so that a differential pressure between the transfer surface and a portion near the vent hole is generated. The differential pressure causes the molding to sink only at the portion near the vent hole, thereby preventing the molding from sinking in the mirror surface.
However, because the prescribed sinking in the surface other than the transfer surface does not occur widely, such a prevention of the sinking in the transfer surface, which is achieved by causing the molding to sink in the surface other than the transfer surface, is confined to a portion near the vent hole. Accordingly, the molding may be caused to sink in a portion far from the vent hole.
Further, still another background mold assembly is disclosed in the Japanese Laid-Open Patent Publication No. 11-28745, which includes a slide cavity piece providing a surface of a cavity other than the transfer surface. A gap is forcibly formed between the slide cavity piece and resin injected into the cavity when the slide cavity piece is slid in a direction away from the resin. Therefore, sinking in the transfer surface is prevented and inward deformation is reduced to a low level.
However, in producing a large molding having high accuracy, it is required to intentionally cause the molding to sink in a large area. Therefore the area of the cavity piece which is brought into contact with the resin is required to be large. In this case, contact-force between the cavity piece and the resin becomes large, which deforms the molding when the cavity piece is forcibly separated from the resin.
Therefore, in producing an uneven, large, or peculiar molding by the above-mentioned methods, a thick portion neighboring the transfer surface may be caused to sink due to differences of the cooling rates. Further, an inward pressure or distortion remains in a thin portion of the molding. Accordingly, the accuracy of the shape is deteriorated. In particular, the accuracy of optical properties may be deteriorated due to an increase of the birefringence of the plastics.